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    <pre><code>WeatherWidget.prototype = new Widget; </code></pre> <p>does create a new instance of the <code>Widget</code> constructor and use it as <code>WeatherWidget</code>'s prototype object. Using the <a href="https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/new" rel="nofollow noreferrer"><code>new</code> keyword</a> creates the new object, sets up the inheritance chain of it to <code>Widget.prototype</code>, and applies the constructor function on it (where you can set up individual properties'n'methods, or create private-scoped variables).</p> <p>Without the <code>new</code> keyword it would be an assignment of the <code>Widget</code> function to the <code>prototype</code> property - which does not make any sense. If you'd add the optional brackets (i.e. <code>Widget()</code>), it would invoke the function normally, but not as a constructor on a new instance, but with the global object as context. See also the <a href="https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/this" rel="nofollow noreferrer">reference for the <code>this</code> keyword</a>.</p> <p>Notice that you should not really use this code. As said, it creates a new instance by invoking the constructor function. But the purpose is only to create an <em>empty</em> object that inherits from the <code>Widget</code>s prototype object, not to instantiate something (which could do some harm, depending on the code). Instead, you should use <a href="https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Object/create" rel="nofollow noreferrer"><code>Object.create</code></a> (or its <a href="https://stackoverflow.com/q/10141086/1048572">popular shim</a>):</p> <pre><code>WeatherWidget.prototype = Object.create(Widget.prototype); </code></pre> <p>see also <a href="https://stackoverflow.com/q/11812648/1048572">Javascript basic inheritance vs Crockford prototypical inheritance</a></p>
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    1. POWhat is the reason to use the 'new' keyword at Derived.prototype = new Base
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    1. COInteresting approach, but please see my updated answer: You would want to fix the `constructor` property as well. And that is where the "popular shim" for `Object.create()` fails – *must* fail even if updated – because you cannot emulate non-enumerability. You can only `hasOwnProperty()`-filter it out later. That still leaves write-onlyness and non-configurability, which can be only partially emulated (depending on the implementation).
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    2. COYes, passing `{constructor:{value:WeatherWidget}}` as the second argument should be done, and you might need to use a less popular shim :-) However, I am not sure which code depends on a correctly set `constructor` property?
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    3. COI have an `isInstance()` method on some of my constructors that does. More precise than duck typing alone.
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